Keep in mind that shorts are frowned on in temples (such as at Angkor Wat). In fact, few men in Cambodia wear shorts unless they have particular sweaty jobs, so there is a class element to this. But since foreigners are seen as completely strange anyway, they can get away with odd behavior and dress to an extent. Certainly lighter dress is fine during exercise (you can go running or biking in the morning along the river in Phnom Penh). Good walking/hiking shoes are a plus for a visit to the temples. Sandals (not leather) are good for rainy season in the city–the mud and fecal matter just rinses right off! Smile: You’ll do this anyway, but always act respectful, don’t raise your voice or your eyebrows, and smile at everybody. Works wonders.

Visa

As of 2003, Visas are available on arrival at the Phnom Penh and Siem Reap airports (see below), so if you are entering Cambodia at the airport, there’s no need to get one beforehand. If you enter by land, you must get a your visa before you get there in most cases, and it must be marked for entry at that entry point. If it isn’t, you are nearly certain to be sent back (Download visa application.)
Find more and better visa info at Tales of Asia.

There is no other preparation needed that I can think of, except for a couple of shots, and for a short visit even those are probably not necessary. Havrix costs $60-100, but is thought to provide lifetime protection from hepatitis A, which is not a bad thing.

Language

The vast majority of Cambodians speak Khmer, a language of the Mon-Khmer group. Its only close relative is the language of the Mon, a Burmese minority. Khmer is only distantly related to Thai and to some Indonesian languages, with some borrowed words from Vietnamese, Chinese, Pali, French and English. The script is related to Devanagari and looks a bit like Thai script at first glance. An increasing number of urban Cambodians speak English, especially young people, and some (mostly older) Cambodians can speak French. Though its grammar is quite straightforward, Khmer is a fairly difficult language for most English speakers to learn because of its pronunciation. For a comparison to other languages, click here. If you want to go beyond the tourist phrasebooks, you can study online at Northern Illinois University’s introduction to Khmer site. For home study, I especially like Frank Smith’s Khmer Language Learning Materials.